source: computer-networks-webdesign.com CSCI 6760 - Computer Networks Spring 2017 Instructor: Prof. Roberto Perdisci perdisci@cs.uga.edu These slides are adapted from the textbook slides by J.F. Kurose and K.W. Ross
Chapter 3: Transport Layer Our goals: } learn about transport layer } understand principles protocols in the Internet: behind transport layer services: } UDP: connectionless transport } TCP: connection-oriented } multiplexing/demultiplexing transport } reliable data transfer } TCP congestion control } flow control } congestion control 3-2 Transport Layer
Transport services and protocols application transport } provide logical communication network data link between app processes running on physical different hosts } transport protocols run in end systems } send side: breaks long app messages into segments, passes to network layer } rcv side: reassembles segments application transport into messages, passes to app network data link layer physical } more than one transport protocol available to apps } Internet: TCP and UDP 3-3 Transport Layer
Transport vs. network layer } network layer: logical Household analogy: communication between hosts 12 kids sending letters to 12 kids } transport layer: logical communication between } processes = kids processes } app messages = letters in } relies on and enhances network envelopes layer services } hosts = houses } transport protocol = Ann and Bill } network-layer protocol = postal service 3-4 Transport Layer
Internet transport-layer protocols application } reliable, in-order delivery transport network (TCP) data link physical network } connection setup data link network physical data link } flow control physical } congestion control network } unreliable, unordered data link physical network delivery: UDP data link physical } extension of “best-effort” IP network data link application physical network } Do not provide: transport data link network physical data link } delay guarantees physical } bandwidth guarantees 3-5 Transport Layer
Multiplexing/demultiplexing Multiplexing at send host: Demultiplexing at rcv host: gathering data from multiple delivering received segments sockets, enveloping data with to correct socket header (later used for demultiplexing) = socket = process P4 application P1 P2 P3 application P1 application transport transport transport network network network link link link physical physical physical host 3 host 2 host 1 3-6 Transport Layer
How demultiplexing works } host receives IP datagrams } each datagram has source IP 32 bits address, destination IP address source port # dest port # } each datagram carries 1 transport-layer segment other header fields } each segment has source and destination port number } host uses IP addresses & port application numbers to direct segment to data appropriate socket (message) TCP/UDP segment format 3-7 Transport Layer
Connectionless demultiplexing } When host receives UDP } Create sockets with port segment: numbers: } checks destination port number DatagramSocket mySocket1 = new in segment DatagramSocket(12534); } directs UDP segment to socket DatagramSocket mySocket2 = new with that port number DatagramSocket(12535); } IP datagrams with different } UDP socket identified by two- source IP addresses and/or tuple: source port numbers ( dest IP address, dest port number) directed to same socket 3-8 Transport Layer
Connectionless demux (cont) DatagramSocket serverSocket = new DatagramSocket(6428); P1 P2 P1 P3 SP: 6428 SP: 6428 DP: 9157 DP: 5775 SP: 9157 SP: 5775 DP: 6428 DP: 6428 client Client server IP: A IP:B IP: C SP provides “return address” 3-9 Transport Layer
Connection-oriented demux } TCP socket identified by 4- } Server host may support tuple: many simultaneous TCP sockets: } source IP address } source port number } each socket identified by its own 4-tuple } dest IP address } Web servers have different } dest port number sockets for each connecting } receiving host uses all four client values to direct segment to } non-persistent HTTP will have appropriate socket different socket for each request 3-10 Transport Layer
Connection-oriented demux (cont) P1 P6 P2 P1 P4 P5 P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 SP: 9157 DP: 80 DP: 80 client Client server IP: A S-IP: A S-IP: B IP:B IP: C D-IP:C D-IP:C 3-11 Transport Layer
Connection-oriented demux: Threaded Web Server P1 P2 P1 P4 P3 SP: 5775 DP: 80 S-IP: B D-IP:C SP: 9157 SP: 9157 DP: 80 DP: 80 client Client server IP: A S-IP: A S-IP: B IP:B IP: C D-IP:C D-IP:C 3-12 Transport Layer
Chapter 3 outline } 3.1 Transport-layer services } 3.5 Connection-oriented } 3.2 Multiplexing and transport: TCP demultiplexing } segment structure } 3.3 Connectionless } reliable data transfer transport: UDP } flow control } 3.4 Principles of reliable data } connection management transfer } 3.6 Principles of congestion control } 3.7 TCP congestion control 3-13 Transport Layer
UDP: User Datagram Protocol [RFC 768] } “no frills,” “bare bones” Internet transport protocol Why is there a UDP? } “best effort” service, UDP } no connection establishment segments may be: (which can add delay) } lost } simple: no connection state at } delivered out of order to app sender, receiver } connectionless: } small segment header } no handshaking between } no congestion control: UDP can UDP sender, receiver blast away as fast as desired } each UDP segment handled independently of others 3-14 Transport Layer
UDP: more } often used for streaming 32 bits multimedia apps source port # dest port # } loss tolerant Length, in checksum bytes of UDP length } rate sensitive segment, } other UDP uses including header } DNS } SNMP Application } reliable transfer over UDP: add data reliability at application layer (message) } application-specific error recovery! UDP segment format 3-15 Transport Layer
UDP checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment Sender: Receiver: } treat segment contents as } compute checksum of received sequence of 16-bit integers segment } checksum: addition (1’s } check if computed checksum complement sum) of segment equals checksum field value: contents } NO - error detected } sender puts checksum value } YES - no error detected. But into UDP checksum field maybe errors nonetheless? More later …. 3-16 Transport Layer
Internet Checksum Example } Note } When adding numbers, a carryout from the most significant bit needs to be added to the result } Example: add two 16-bit integers 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 3-17 Transport Layer
Principles of Reliable data transfer } important in app., transport, link layers } top-10 list of important networking topics! } characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt) 3-18 Transport Layer
Principles of Reliable data transfer 3-19 Transport Layer
Principles of Reliable data transfer 3-20 Transport Layer
Reliable data transfer: getting started rdt_send(): called from above, deliver_data(): called by (e.g., by app.). Passed data to rdt to deliver data to upper deliver to receiver upper layer send receive side side udt_send(): called by rdt, rdt_rcv(): called when packet to transfer packet over arrives on rcv-side of channel unreliable channel to receiver 3-21 Transport Layer
Reliable data transfer: getting started We’ll: } incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) } consider only unidirectional data transfer } but control info will flow on both directions! } use finite state machines (FSM) to specify sender, receiver event causing state transition actions taken on state transition state: when in this state state “state” next state event 1 uniquely determined 2 actions by next event 3-22 Transport Layer
Rdt1.0: reliable transfer over a reliable channel } underlying channel perfectly reliable } no bit errors } no loss of packets } separate FSMs for sender, receiver: } sender sends data into underlying channel } receiver read data from underlying channel rdt_send(data) rdt_rcv(packet) Wait for Wait for call from call from extract (packet,data) packet = make_pkt(data) below above deliver_data(data) udt_send(packet) receiver sender 3-23 Transport Layer
Rdt2.0: channel with bit errors } underlying channel may flip bits in packet } checksum to detect bit errors } the question: how to recover from errors: } acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK } negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors } sender retransmits pkt on receipt of NAK } new mechanisms in rdt2.0 (beyond rdt1.0 ): } error detection } receiver feedback: control msgs (ACK,NAK) rcvr->sender 3-24 Transport Layer
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